Gold's big rally has sparked a bearish stock-market signal that hasn't been seen in over 20 years

Gold's rally is breaking down a long-running relationship the metal has with stocks, Stifel wrote in a note.

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• The S&P 500's relationship to gold is flashing a worrying signal, Stifel strategists said.
• The index's price relative to gold recently fell below its seven-year moving average.
• That hasn't happened since 2002, and has historically been followed by a challenging period for stocks.

The stock market is flashing...